Rechargeable batteries have experienced a wide range of usage in today's society. Rechargeable automotive batteries have traditionally been used in automobiles, while sealed batteries or cells such as nickel cadmium batteries are also widely used.
In order to recharge said batteries several different kinds of chargers have heretofore been proposed and utilized. Such chargers may be broadly classified as simple chargers or feedback chargers. Simple chargers basically function without feedback, that is without monitoring the state of the battery and adjusting the charging current in response thereto.
Since rechargeable batteries tend to produce gases internally on overcharging capable of rupturing the battery, and also overheat and damage the battery upon overcharging, such simple chargers have traditionally been constructed to deliver a low level charge as modern day cell construction is capable of withstanding even prolonged overcharging provided such overcharging rate is maintained at a low level.
Accordingly, simple chargers have been constructed to:
(a) deliver a constant low current (sometimes referred to as trickle chargers) to prevent damage to the battery due to over charging, or; PA0 (b) charge over a timed interval. PA0 (a) provide high charge currents during the first part of the charge cycle; and PA0 (b) reduce and eventually stop charging when full charge has been reached. PA0 (a) open cell potential, PA0 (b) the electrochemical component of the overvoltage, plus PA0 (c) the resistive component of the overvoltage.
Furthermore, simple chargers have been constructed to deliver a constant voltage charge whereby the charge current is reduced to zero or trickle as the battery voltage rises. Moreover, simple chargers have been modified to increase the internal charger resistance to provide a "taper" characteristic whereby the charge current gradually tapers down to a trickle as the battery voltage rises.
However, such simple chargers provide a battery with a non-optimal charge profile and thereby provide conservative charge rates resulting in long charge times.
More sophisticated chargers commonly referred to as feedback chargers or closed loop chargers employ some means of monitoring the state of charge of the battery in order to:
Such chargers vary in design according to the type and size of the battery to be charged, degree of sophistication and other factors. Such chargers use various monitored battery parameters such as cell voltage, cell temperature, charge time or a specific feature such as temperature rise or rate of voltage rise to effect control of the charge current in a feedback loop configuration.
For example, U.S. Pat. No. 3,531,706 discloses a charger which supplies a battery with an initial rapid charge rate followed by a trickle charged current in response to the cell temperature and cell voltage of the battery.
Feedback chargers used in the past have monitored the overall cell voltage which consists of the sum of the:
Feedback chargers which monitor the overall voltage of a battery that includes the resistive component of cell overvoltage, or the open cell potential, yield unreliable results as such voltages vary with the age history of the battery, that is the number of times or lengths of time over which a battery has been discharged, recharged, overcharged, deep discharged or damaged.
Internal resistance free method in testing fuel cells and batteries have been described in various articles including K. Kordesch and A. Marko "Sine Wave Pulse Current Tester for Batteries", J. Electrochem Soc. Vol. 107, pages 480-83 (1960). This method by virtue of eliminating the resistive component of the cell overvoltage caused by the cell internal resistance, allows measurement of the electrochemical cell potential, which as described above is the sum of the open cell potential and the electrochemical component of overvoltage. This electrochemical cell potential provides a good indication of the internal state of the cell, related to the type of chemistry of the cell, passing Current and electrochemical performance of its elements, but not affected by the resistance of conductors, electrodes and electrolyte.
In order to obtain reliable internal resistance free potential readings, certain precautions must be followed, as particularized in the following paragraphs.
After the abrupt termination of the current pulse, cell voltage tends to the open circuit voltage over a period of time. This complex transient is characterized by several time constants. The first and relatively fast transient, is the electrical transient, related to the electrical resistance, capacitance and inductance of the conductors, electrodes, and electrolyte. This time constant is typically in the range of microseconds to hundreds of microseconds. The second transient which is related to the electrochemical polarization (concentration polarization, transport phenomena, gassing etc.) is much slower, with time constants typically in the range of 100-1000 milisecond or even longer. It is important, for proper and precise detection of the internal resistance (IR) free potential, to take the reading after the electrical transient has safely died out, and before the equilibrium of the electrochemical process had time to change. From our experience, proper delay is in the range of 0.5 to 10 ms, while for most small "flashlight" cells the best value is about 1-3 ms.
A strobing technique which extracts the value of the cell potential at this predetermined exact time after the abrupt interruption but disregards the cell potential before and after this exact moment yields reliable internal resistance free potential readings.
It is an object of this invention to produce a feedback charger which is more efficient, safe and reliable than previously used.
It is a further object of this invention to produce a feedback charger with electrical characteristics controlled by feedback derived from the internal resistance free electrochemical potential of the battery.